Gotcha. I suspect you should have had it installed on yours. Run your VIN through http://carinfo.kiev.ua/cars/vin/mercedes and check the installed options report for:
R65 SPARE WHEEL CARRIER UNDER FRAME END
R87 SPARE WHEEL
Does the set up look something like this?https://www.ebay.com/i/132636842944?chn=ps
That seems overkill but trying to get an understanding of what I need to put in there.
Shoot I ordered the underneath carriage, installed it but there are 2 points welded to the body not allowing the tire to fit. Thinking of sawing them off since they don’t appear to do anything. They almost look like they were installed there for a larger generator or water tank.